thermal: cpu_cooling: get rid of 'allowed_cpus'

'allowed_cpus' is a copy of policy->related_cpus and can be replaced by
it directly. At some places we are only concerned about online CPUs and
policy->cpus can be used there.

Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Reviewed-by: Lukasz Luba <lukasz.luba@arm.com>
Tested-by: Lukasz Luba <lukasz.luba@arm.com>
Signed-off-by: Eduardo Valentin <edubezval@gmail.com>